Method for fabricating pressure sensitive sevices



United States Patent METHOD FOR FABRICATING PRESSURE SENSITIVE DEVICES William R. Polye, River Edge, N. J., assignor to Bendix Aviation Corporation, Teterboro, N. J., a corporation of Delaware Application May 25, 1953, Serial No. 357,280 9 Claims. (Cl. 536) This invention relates in general to a method for fabricating pressure sensitive devices and more particularly to a novel method for fabricating high vacuum diaphragms.

This application is a division of applicationSerial No. 281,435, tiled April 9, 1952, now Patent Number 2,718,904, in the name of William R. Polye.

High vacuum diaphragms are employed in altimcters intended for use at very high altitudes because these diaphragms must 'be responsive to very small ambient atmospher-ic pressures. Generally, the pressure within the evacuated diaphragm is to be maintained somewhere between 10- to 10- mm. of mercury.

Heret-ofore, high vacuum diaphra-gms were exhausted by a suction pump to the desired vacuum and sealed-off. 'In operation, it was :found that altimeters employing diaphragms so constructed were subject to error because of the residual gases present after completion of the exhaustion operation. Thus, the diaphragms became sensitive to temperature changes as well as to pressure changes because the residual gases expanded with changes in ambient temperatures. The residual gases comprised the gases released from the solder flux and from the metal parts which gave rise to an increase in internal pressure after sealing oii. It is well known that metal surfaces have the ability to retain a small amount of gas therein and consequently, the process of evacuating the diaphragm does not efiectively remove all of the gas.

An object of the invention, therefore, is to provide a novel method for fabricating high vacuum diaphragms wherein the interior of the diaphragm is connected to a chamber containing an adsorbent material. The diaphragm and chamber are simultaneously evacuated and heated with the latter being heated to a higher degree than the former and the adsorbent material is lefit in the chamber after the evacuation operation and the sealing of the chambers to adsorb occluded gases.

The above and other objects and advantages of the present invention will appear more :fully hereinafter from a consideration of the detailed description which follow-s taken together with the accompanying drawing wherein the single figure thereof illustrates one embodiment of the invention.

Referring now to the drawing for a more detailed de- 'scription of the present invention, a pressure sensitive assembly is shown as comprising a metal diaphragm 12, consisting of a pair of corrugated flexible discs 14 and 16 joined together along their peripheries by :a suitable solder, as at 1:8, to form an expansible and contract-ible chamber or casing. Disc 14 is provided with a log 20 to Whichany suitable transmission mechanism may be connected when the assembly is installed within a device such as an altimeter. Disc 16, on the other hand, is provided with a lug 22 having an opening therein register ing with an opening centrally formed in the disc. A hollow metal tube 24 is mounted at one end to extend into the opening in lug 22 where it is secured by suitable means such as solder. The other end of t be '24 is mounted through a glass to metal seal 26 to an elongated glass bulb or envelope 28 having its interior tilled with a gas adsorbing material 30. Seal 26 is fabricated from a suitable alloy of chromium, nickel and iron available on the market. Thus, tube 24 and seal 26 term means for communicating the interior of envelope '28 with the interior of diaphragm .12.

Gas adsorbing material 30 comprises activated cocoanut charcoal granules, although other materials such as silica-gel or activated alumina may be used. Adsorbing material 30 has the characteristic of adsorbing gas when subjected to dropping temperatures and expelling or giving-off the gas when heated. The granules are capable of adsorbing large volumes of gas and for this reason, they eifectively adsorb the residual gases in the diaphragm.

After the discs 14 and 16 are soldered together and bulb 28 and tubes 26, 24 are assembled thereto, a suction pump :32 is connected through a conduit '34 to the interior of bulb 28 at the portion where the bulb is to be tipped-01f, and the assembly is evacuated. A domeshaped oven 36 which may be heated, for example, by heating elements 38 connected in a heating circuit comprising leads 40 and 42, battery 44 and switch 46, is placed over the assembly before evacuation to heat the latter during the exhaustion process whereby the gas within the diaphragm is substantially drawn therefirom; the gas in this case being partially adsorbed by material 30 and partially taken out by pump 32. The oven 36 is brought to a temperature below the melting point of solder 18, which runs about 350 degrees Fahrenheit, to prevent melting of the solder joints. Since the temperature to which diaphragm 12 is brought is limited by the melting point temperature of solder '18, a very small amount of occluded gas will not be drawn out and will remain in diaphragm 12. Toward the end of the exhaustion operation, a heater coil =48 which may be fabricated of an alloy of nickel and chromium available on the market, and which is heated, for example, through a suitable electrical circuit, comprising leads '50 and '52, battery 54 and switch 56 to drive out the gases already adsorbed by the adsorbing material 30. Coil 48 is brought to a temperature higher than that of oven 36, for example, at approximately 600 degrees Fahrenheit.

Since the temperature at which solder 1'8 melts is about 350 degrees Fahrenheit, the diaphragm 12 may be insulated from the bulb, if necessary, as by sheets of asbestos 5'8 paced between diaphragm 12 and bulb '28. Bulb 28 is made of glass in order to prevent undue trans-fer of heat to the diaphragm, glass being a relatively poor conductor of heat. v

The bulb 28 is then tipped-off adjacent the bulb in a manner well-known in the art and the pump 32 is disconnected. The material 30 cools from its outgassi-ng temperature to room temperature and, in cooling, adsorbs the very small amount of gas remaining in diaphragm'12. The above action will continue to take place until equilibrium is established; however, the adsorbent 30 is capable of adsorbing such large volumes of gas that equilibrium is usually not reached unless the diaphragm assembly leaks. The diaphragm assembly now is capable of maintaining a low pressure in diaphragm 12.

It will now be readily apparent that the present invention provides a novel method for fabricating a high vacuum diaphragm which operates efliciently over a wid range of temperatures and pressures. The occluded gas within the metal parts and other gas released from the solder flux are substantially driven out of the diaphragm by heating of the latter, and a low pressure is maintained in the diaphragm after sealing off by the gas adsorbing material.

Although on embodiment of the present invention has been illustrated and described in detail, it is to be eX- pressly understood that the invention is not limited thereto and that various changes and modifications can be made without departing from the spirit and scope of the invention as the same will now be understood by those skilled in th art.

What is claimed is:

1. The method of producing and maintaining a high vacuum in a pressure sensitive diaphragm in communication with an envelope containing a gas adsorbing material, comprising the steps of heating the diaphragm and envelope, heating the envelope to a higher temperature than the diaphragm to increase the temperature of the gas adsorbing material to drive d the gases adsorbed by said material, evacuating the diaphragm and envelope during the heating thereof, and sealing offsaid envelope when the desired vacuum is obtained.

2. The method of producing and maintaining a high vacuum in a soft soldered corrugated metal pressure sensitive diaphragm comprising the steps of connecting the diaphragm to an envelope, filling the envelope with gas adsorbing material, heating the diaphragm and envelope to a temperature below the melting point of the solder, thereafter heating the envelope at a higher temperature than the heating of the diaphragm to drive off the gases adsorbed by said material, evacuating the diaphragm and envelope during the heating thereof, and sealing off said envelope when the desired vacuum is obtained.

3. The method of producing a high vacuum in a soft soldered corrugated metal pressure sensitive diaphragm in communication with an envelope containing gas adsorbing material, comprising the steps of heat insulating the diaphragm from the envelope, heating the diaphragm and envelope to a temperature below the melting point of the solder, thereafter heating the envelope to a temperatur above the first temperature at which the dia phragm and envelope are heated, and sealing off the envelope when the desired vacuum is obtained.

4. The method of producing and maintaining a high vacuum in an expansible and eontractible pressure sensitive device, comprising the steps of permanently communicating the device with a container having a gas adsorbing medium therein, evacuating the device and container whereby all gases except residual gases are removed therefrom, heating the gas adsorbing medium during evacuation to drive adsorbed gases therefrom, and thereafter sealing the container whereby the gas adsorbing medium adsorbs the residual gases.

5. The method of producing and maintaining a high vacuum in a hollow casing in communication with an envelope containing a gas adsorbing material, which comprises heating the casing and envelope in an oven to a selected temperature, and thereafter heating the envelope alone to a higher temperature within the oven and thereby driving otf gases substantially at said material while said casing is maintained in said first-mentioned temperature, evacuating the casing and envelope during heating thereof, and hermetically sealing said envelope and easing when the desired vacuum is obtained.

6. The method of producing and maintaining a high vacuum in a hollow casing in communication with an envelope containing a gas adsorbing material, which comprises heating the casing and envelope in an oven to a selected temperature, thereafter heating the envelope alone to a higher temperature with a separate source of heat located within the oven and outside the envelope,

thereby driving off gases in said material, evacuating the casing and envelope during heating thereof, and hermetically sealing said envelope and casing when the desired vacuum is obtained.

7. The method of producing and maintaining a high vacuum in a casing including a resilient portion soldered thereto and being in communication with an envelope containing-a gas adsorbing material, which comprises heating said casing and said envelope simultaneously to a first temperature below the melting temperature of the solder, thereafter heating the envelope to a second tem perature above the melting temperature of the solder and thereby driving otf gases in said material while maintaining the solder below the melting point thereof, evacuating the casing and envelope during heating thereof, and hermetically sealing said envelope and easing when the desired vacuum is obtained.

8. The method of producing and maintaining a high vacuum in a hollow casing in communication with an envelope containing a gas adsorbing material, which comprises heating the hollow casing and envelope to within a selected first temperature range, and thereafter heating the envelope to a higher second temperature and thereby driving off gases in said gas adsorbing material while said casing is maintained at a temperature not greater than the highest temperature within said first temperature range, evacuating the casing and envelope at least during heating of said envelope at said second temperature, and hermetically sealing said envelope and easing when the desired high vacuum is obtained.

9. The method of producing a pressure sensitive device having a chamber portion including a resilient element and an envelope portion containing a gas adsorbing material, comprising the steps of heating said chamber portion and said envelope portion to within a first predetermined temperature range, thereafter heating only the envelope portion to within a second predetermined temperature range which is higher than said first predetermined temperature range for further driving off gases While maintaining the temperature of the chamber portion substantially within said first predetermined temperature range, evacuating the chamber and envelope portions at least during a part of the heating of said envelope portion, and hermetically sealing said chamber and envelope portions when a desired vacuum therewithin is obtained.

References Cited in the file of this patent UNITED STATES PATENTS 845,670 Thomas Feb. 26, 1907 1,124,555 Thatcher Jan. 12, 1915 1,146,019 Pfund July 13, 1915 1,551,527 Pirani Aug. 25, 1925 

